diff --git a/libavformat/asfdec_f.c b/libavformat/asfdec_f.c
index e9ddca7151..103155e9e7 100644
--- a/libavformat/asfdec_f.c
+++ b/libavformat/asfdec_f.c
@@ -424,7 +424,7 @@ static int asf_read_stream_properties(AVFormatContext *s, int64_t size)
     if (!(asf->hdr.flags & 0x01)) { // if we aren't streaming...
         int64_t fsize = avio_size(pb);
         if (fsize <= 0 || (int64_t)asf->hdr.file_size <= 0 ||
-            20*FFABS(fsize - (int64_t)asf->hdr.file_size) < FFMIN(fsize, asf->hdr.file_size))
+            FFABS(fsize - (int64_t)asf->hdr.file_size) < FFMIN(fsize, asf->hdr.file_size)/20)
             st->duration = asf->hdr.play_time /
                        (10000000 / 1000) - start_time;
     }